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Concrete negative effect mechanism of polycarboxylate water reducer and clay

- Mar 31, 2018 -

The reason why the clay has a great negative effect on the work performance of water-reduced concrete is mainly related to the clay layer structure and large specific surface area, which leads to its strong adsorption. On the one hand, clay adsorbs a large amount of water-reducing agent, which reduces the effective water-reducing agent content of the water-reducing dispersion, resulting in a decrease in the flowability of the concrete; on the other hand, the clay adsorbs a large amount of mixing water, which is provided in the concrete mixture. The free water of fluidity is reduced, thereby reducing the fluidity of the concrete. The negative effect of clay on water-reducing agent concrete is the result of the joint action of clay adsorption water-reducing agent and adsorption and mixing water, and which one of the main effects may be related to the type of water-reducing agent.

I. Adsorption of Polycarboxylate Water Reducing Agent by Clay

Sakai et al. used TOC to measure the adsorption amount of polycarboxylic acid water-reducing agent by montmorillonite and stone powder, indicating that the adsorption amount of polycarboxylic acid water-reducing agent by montmorillonite far exceeds the adsorption amount of polycarboxylic acid water-reducing agent by stone powder. The research results of Ma Baoguo et al. showed that the adsorption rate of polycarboxylate water reducer in soil was higher than that of cement to water reducer. The adsorption amount increased with time and eventually reached equilibrium. Cement and soil were used for carboxylation. The saturated adsorptive capacity of the acid water-reducing agent upon adsorption equilibrium was 3.7 mg/g and 10.1 mg/g, respectively. Wang Ziming et al. used TOC to study the adsorption capacity of polycarboxylate superplasticizers by cement and different clays. The results showed that the adsorption amount of polycarboxylate superplasticizer for cement was about 2~4 mg/g, and the kaolin was used for polycarboxylate superplasticizers. The adsorption capacity is approximately 5 times the adsorption capacity of the cement particles to the polycarboxylic acid water reducer, and is approximately 10 to 20 mg/g. The adsorption amount of the polycarboxylate water reducing agent by the bentonite is approximately the cement particle to the polycarboxylic acid water reducing agent. Adsorption capacity is 50 times, about 110~120mg/g; In addition, the adsorption amount of polycarboxylate superplasticizer by kaolin and bentonite does not change much with time, indicating that the adsorption of polycarboxylate superplasticizer by clay is very early. Saturated adsorption capacity has been reached in a short time. Wang Zhi et al. measured the amount of polycarboxylate superplasticizer adsorbed by cement and the four clays of sodium-based montmorillonite, calcium-based montmorillonite, illite, and kaolin under the same conditions. The experimental results show that cement reduces polycarboxylic acid. Adsorption capacity of water agent is 37mg/g; adsorption capacity of sodium montmorillonite and calcium montmorillonite to polycarboxylic acid water reducer is 75mg/g and 95mg/g, respectively; Yilituo to polycarboxylic acid water reducer The adsorption amount was 41 mg/g; the adsorption amount of kaolin to the polycarboxylic acid water reducing agent was 40 mg/g. Some researchers may have some experimental results related to the test method and test specific conditions, but it can be seen from the overall trend that clay is more likely to absorb polycarboxylate water reducer than cement, and the adsorption capacity is also greater.

Although the adsorption amount of polycarboxylic acid water-reducing agent by different clays can explain its degree of negative effect on the polycarboxylic acid water-reducing agent, it does not reflect how the polycarboxylic acid water-reducing agent is adsorbed by the clay. One view is that clay is a polycarboxylate water-reducing agent that is adsorbed by surface adsorption. The charging properties of clay particles are affected by the pH value of the solution. When the solution is acidic, the clay particles are generally positively charged, and when the solution is alkali In the case of sex, clay particles are negatively charged, so clay particles are usually negatively charged in the alkaline environment of cement concrete systems. This negative charge has a repulsive effect on the anion RCOO-, etc., in the polycarboxylic acid water reducer and is not conducive to adsorption. Therefore, it is unreasonable that the surface of the clay particle absorbs a large amount of the polycarboxylic acid water reducer through the charge. Although the clay is generally negatively charged in the alkaline environment of cement concrete, the charge distribution is not uniform, and a small amount of polycarboxylic acid water reducer may also be adsorbed by the electrostatic force in the local area. Clay can also form an intermediary by adsorbing calcium ions on the negative surface of the clay, thereby adsorbing polycarboxylic acid water reducer molecules. In addition, because clays have a large surface energy, it is also possible to absorb polycarboxylic acid water reducers by van der Waals forces. Another view is that clays are mainly polycarboxylate superplasticizers adsorbed by interlayer adsorption. Theoretical calculations show that the interlayer spacing of sodium-based montmorillonites is 12.50 angstroms, and the interlayer spacing of calcium-based montmorillonites is 15.50. Egypt, whether it is sodium montmorillonite or calcium montmorillonite, polycarboxylic acid water reducer molecules can not enter the montmorillonite layer. However, the longitudinal radius of the polycarboxylic acid water-reducing agent side chain after being folded and agglomerated in water is about 0.4~1 nm, and it can be adsorbed into the clay layer in size. Plank's XRD test found that the spacing of the sodium-based montmorillonite layer after adsorption of the polycarboxylate water-reducing agent rose from 1.23 nm to 1.77 nm. He believes that the clay-adsorbing polycarboxylate water-reducing agent molecules are intercalated into the interlayers of the side chains. It is not the case that the whole body enters the interlayer; Wu et al.'s study also found that the spacing of the montmorillonite layer treated with the polycarboxylate water reducer increases from 1.487 nm to 1.863 nm; Wang Zhi et al. study shows that the montmorillonite and the ether are aggregated. The interlayer spacing after the action of the carboxylic acid water reducer is increased from 1.49 nm to 1.75 nm, and the interlayer distance after the interaction with the ester polycarboxylic acid water reducer is enlarged to 1.83 nm, from which the polycarboxylic acid water reducer can be inferred. The molecules are indeed adsorbed between clay layers.

二、Adsorption of mixing water by clay

The mechanism of most of the scholars' influence on the clay-based polycarboxylate water-reducing agent is mainly explained by the amount of cement and clay adsorbed on the polycarboxylate water-reducing agent. It may also affect the workability of the concrete by only absorbing the mixing water from the clay. The above analysis gives a qualitative description and few experimental verifications. Hu Qianwen studied the difference in water absorption of different clays through the clay swelling capacity test and fluidity test, and explored the mechanism of the influence on the fluidity of concrete from the viewpoint of the clay water absorption performance. The results of the study indicate that different swelling capacities of different clays are different from that of sodium-based montmorillonite> calcium-based montmorillonite> illite> kaolin. Sodium montmorillonite and calcium montmorillonite have strong water swelling properties. Under the condition of ensuring 250 mm fluidity, the water absorption capacity of sodium montmorillonite is 8 g/g, and that of sodium montmorillonite is 8 g/g. The amount of water absorbed by montmorillonite is 1.7 g/g, and the water absorption of illite and kaolin is weak.